Flashcards
System Admin- DHCP Service
Flashcards do conjunto (30)
- 1What does DHCP stand for?DHCP is short for Dynamic Host Configuration Protocol
- 2What is the DHCP be able to do?able to provide IP configuration information for hosts on a network, such info can be as simple as an :
- IP address
- subnet mask
- Default Gateway - 3what is an major advantage of using DHCP on even a small network?From the users perspective it offers a "set and forget" approach to IP addressing, providing that whoever is setting up the server knows what they are doing.
- 4IPv4 vs IPv6in an IPv4 world:
-clients need configuring to enable them to work on routed networks
- IP addresses are a scarce commodity and so their use needs careful management
In an IPv6 world:
- clients are capable of Stateless Address auto-configuration (SLAAC), so they can give themselves a routeable IP address
- there are so many IPv6 IP addresses (approx 667x10(21)) per square metre. that managing them is not an issue.
-DHCP is seen as being for connecting devices not clients - 5DHCP is a ---------- process?client/server
- 6For a host to use DHCP what must be configured?automatic configuration of it's IP set up, for practical reasons this tends to be the out-of-the-box configuration of many hosts
- 7What are the 3 modes of operation of a DHCP server?DYNAMIC
- IP configuration is assigned to the client for a fixed period of time (lease period).
- If the lease is not renewed during this period the server can reclaim the IP address and re-allocate it to another host/client.
AUTOMATIC
- Like dynamic but the IP configuration is permanently assigned to a client and the server maintains tables of assignments so it can preferentially assign a previous address to a returning client.
STATIC
- on the DHCP server, specific IP addresses are associated with specific MAC addresses
- Useful for network servers, whose IP addresses should not change, aka DHCP Reservation or fixed-address allocation
A client may also have its IP configuration set up manually by the user - 8Stages in DHCP address allocation: Stage 1: DHCP Discovery-if a client is set up to use DHCP but has no record of any previous DHCP config then it has to find a DHCP server to give it configuration
- it does this by sending a DHCPDISCOVER packet out to the network
-As it has no idea of the IP address of potential DHCP servers, it has to send out a BROADCAST request/packet i.e. the destination IP address is 255.255.255.255. In the source address field it puts an IP address of 0.0.0.0 as it does not have an address of it's own yet.
- if the DHCP server is on a different subnet then the intervening routers must be configured to RELAY the packet to the DHCP server. - 9Stage 2: DHCP Offer-All DHCP servers who receive a DHCPDISCOVER packet from a client take a loot at the packet. what they need to consider is:
- Do I have a pool of addresses suitable for the subnet to which this client is attached?
- If I do, do I have an address available?
-if YES to both above, the DHCP server temporarily reserves an IP address and sends out a DHCPOFFER packet to the client which includes:
- offered IP address
-associated subnet mask
As the client doesn't have an IP address yet, this has to be another broadcast packet but the server can put it's own IP address in the source IP address field - 10Stage 3: Client accept-The client accept's the first offer it receives
-as more than 1 DHCP server may have made an offer the DHCPREQUEST packet that the client sends to accept the offer is still a broadcast packet as it's the most efficient way of ensuring all DHCP servers know which offer has been accepted.
-the client now has an IP address it puts this in the source address field
- The acceptance is broadcast so all DHCP servers that made an offer receive it. The offers not accepted will be put back into the pool of available addresses for that DHCP server. - 11Stage 4: DHCP AcknowledgementThe DHCP server whose offer was accepted sends an acknowledgement back to the client.
- this is the only unicast message in the whole process
- in the DHCPACK packet as well as confirmation of the IP address it also sends lease duration and any other IP configuration information the server is supplying. - 12What are the two ways the renewal process can be done?On restart and Periodically
- 13On Restart- when clients are restarted they contact the DHCP server to renew their IP address
- as this is a renewal of an existing IP address, this is a unicast DHCPRENEW packet from the client to DHCP server from which they got their address
- If the server is up you can just send back a DHCPACK confirming the IP address, and a new lease expiry date
- if the server cannot be contacted but the current lease is still in date the client continues to use its current configuration
-if the lease does eventually run out then the client has to go through the whole leasing process from the beginning (four stage acquisition process) - 14Periodically- clients that are up all he time will go through a renewal process part way through the period of the lease (half way)
- if the lease cannot be renewed they will try again closer to the time (3/4 of the lease period)
-if the lease cannot be renewed then the client has to go through the whole leasing process from beginning - 15What is The release process?release of an allocated IP address lets a DHCP server know that this address is no longer being used on the network and it can allocate it to another client.
- 16What are 2 ways of releasing an IP address?Automatically and Manually
- 17AutomaticallyWhen a client is shutdown gracefully it sends a DHCPRELEASE packet to the server
- 18Manuallywithin the ipconfig utility there is a functionality for formally releasing an IP address back to a DHCP server.
-useful because some clients are not clued into the automatic release process on shutdown and so hang on to the IP addresses even when not using them
-the release process is not a mandatory part of the protocol as clients are not always gracefully shutdown - 19A DHCP server will always provide the following config information.....IP address
subnet mask
lease duration - 20single and multiple DHCP serversSingle DHCP server will allocate unique addresses to each client
Multiple DHCP servers may be used for fault tolerance & load balancing.
care must be taken to ensure that they cannot allocate the same address to different clients- each server must have a unique set of IP addresses that it can use - 21DHCP server may also provide much additional info to a client:Router (Default Gateway)
Time Server
Domain Name Server
Domain Name
Default IP Time to Live
Static routes
Mail Server
A client may also obtain vendor specific info - 22What is a DHCP Scope?A pool of consecutive IP addresses for a subnet that can be allocated to clients on that subnet
- 23Types of scopesA given server may be responsible for a single scope or multiple scopes
- 24When a scope is created the following info is associated with it:NetID
Subnet Mask
IP address range
Lease Duration
Scope Name
Exclusion Range(optional)
Options (Optional) - 25Options are additional info given to clients, options may be linked to different level within the DHCP server such as?SERVER
Options linked at this level apply to all scopes defined on server
SCOPE
Options linked at this level apply to the specified scope and take precedence over any options defined at the server level
CLASS
options linked at this level apply to the specified vendor class and take precedence over any options defined at server and scope level
RESERVATION
A reservation assigns a specific IP address to an NIC with a specific MAC address. options linked at this level apply to the specified reservation and take precedence over any options defined at server, scope and class level. - 26Tutorial Q1: what is the advantage of using a DHCP server to allocate IP addresses and other config info to client on a network?If a DHCP server is used then the configuration for a whole subnet has to be only set up once rather than individually on each client. If a client is moved to a new subnet it will automatically get the correct allocation for that subnet
- 27Q2: what are the 3 modes of operation of a DHCP server and what is the difference between them?DYNAMIC
IP configuration is assigned to the client for a fixed period of time (lease period).
- If the lease is not renewed during this period the server can reclaim the IP address and re-allocate it to another host/client.
AUTOMATIC
- Like dynamic but the IP configuration is permanently assigned to a client and the server maintains tables of assignments so it can preferentially assign a previous address to a returning client.
STATIC
- on the DHCP server, specific IP addresses are associated with specific MAC addresses
- Useful for network servers, whose IP addresses should not change, aka DHCP Reservation or fixed-address allocation - 28Q3: Why are three of the four packets in DHCP address allocation sent to broadcast addresses?- the DHCPDISCOVER packet is sent to a broadcast address because the client does not know the IP address of a DHCP server.
- the DHCPOFFER packet is sent to a broadcast address because the client does not yet have an IP address
- the DHCPREQUEST packet is sent to a broadcast address because this is the most efficient way for a client to let DHCP servers know which offer it is accepting - 29Q4: You shut down your computer on Friday. When you restart it on Monday, the DHCP server from which it got its IP address on Friday cannot be contacted. The lease expires at 9.06am on the coming Friday. What will your computer do?the client still has the IP address which is "in date" so it will continue to use that address until the lease expires
- 30Q5: You are setting up a scope with an IP address range of 172.16.0.1 to 172.16.0.254. When you try to exclude an IP address of 172.16.1.23 from the list of IP addresses that the server can allocate from this scope, you get a complaint. What is wrong with what you are trying to do?the address you are trying to exclude is not within the range of addresses that you have given to the scope to allocate
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- FlashCards sobre System Admin- DHCP Service, criado por Robena786 em 17-05-2015.
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- Robena786
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